1. Field of the Invention
The present invention relates to a master cylinder which is used with a hydraulic brake or clutch system for an automobile. More particularly, the present invention relates to a master cylinder of the portless type which has a piston provided with a valve mechanism for establishing or fluid interrupting communication between a pressure chamber and a reservoir.
2. Description of the Related Art
A conventional master cylinder of the portless type is disclosed, for example, in U.S. Pat. No. 4,707,989. Therein, a piston is fitted slidably in a liquid-tight fashion in a bore of a cylinder body and defining therein a pressure chamber normally in communication with fluid conduits leading to wheel cylinders and a supply chamber normally in communication with a reservoir. The piston has a recess at one end thereof facing the pressure chamber. A valve seat is provided at a bottom of the recess. A valve member is provided in the recess and is movable axially of the piston away from, or onto, contact with the valve seat for establishing or interrupting communication between the pressure chamber and the supply chamber (the reservoir). A first compression spring is provided in the recess for urging the valve member toward the valve seat. A supporting member is connected to the valve member and positioned at the bottom of the pressure chamber. A retainer is fitted on the end of the piston. A connecting rod extends through the supporting member and the retainer and is movable by a predetermined distance axially of the piston. A second compression spring is disposed between the supporting member and the retainer for urging them to move the valve member away from the valve seat by a predetermined distance. The retainer is movable in normal operation axially by a certain distance relative to the piston so as to increase the distance between the valve member and the valve seat and to allow release of any residual fluid in the pressure chamber. The supporting member is fixed to the cylinder body.
In the master cylinder of the portless type, if the brake pedal is again depressed before it returns to its original position, hydraulic fluid causes a cup member to flex and permits hydraulic fluid to flow into the pressure chamber. This fluid produces in the pressure chamber a residual pressure which prevents smooth restoration of the brake pedal to its original position. Further, it is often the case that the residual pressure produced in the pressure chamber as hereinbefore described may cause the valve member to be stretched elastically and stay at rest on the valve seat, even when the piston is brought back to the inoperative position. If the valve member remains closed, the residual pressure cannot be released from the pressure chamber, and disables the brake system completely. This problem may be solved if there is a greater distance between the valve member and the valve seat when the piston is in its inoperative position. This solution, however, results in an increased idle stroke of the piston and therefore of the brake pedal.
One way of overcoming this problem in the above master cylinder has been developed. Since the piston slides relative to the retainer in the direction which the valve member is separated from the valve seat by the residual pressure when the piston is brought back to its inoperative position, the valve member is separated from the valve seat more than the predetermined distance. Therefore, the above problem may be overcome without increasing the idle stroke of the brake pedal as mentioned above.
In the prior master cylinder, however, it is often the case that the piston is caused to move further due to the residual pressure after separating the valve member from the valve seat. If the piston slides more than a certain distance relative to the retainer, large stresses are applied to the retainer, the connecting rod and the supporting member, respectively, and may cause damage to these parts. Furthermore, when these parts are damaged and the piston slides an increased distance toward the supply chamber, a primary cup member passes a supply port providing fluid communication between the supply chamber and the reservoir. As a result, the primary cup member is damaged and braking operation becomes an impossibility due to the loss of the sealing function of the pressure chamber due to the primary cup. Furthermore, an inner surface of the bore of the cylinder body is damaged so as to prevent reuse of the cylinder body. These above drawbacks are not only caused by the residual pressure problem but also are caused when an anti-skid control device or a traction control device of the type which causes hydraulic fluid to flow back from the wheel cylinders to the pressure chamber in the master cylinder through a pump when the wheels are locked or slipping.
The above drawbacks may be solved if the retainer, the connecting rod and the supporting member are increased in strength by increasing size, strength of materials, etc. This solution, however, results in an increased manufacturing cost of the master cylinder and increased weight of the master cylinder.